Define field, element

We use "field" and "element" quite a lot but don't actually define what they are. We need to define them.

We could get rid of the use of "element" (in arrays and tuples) as a separate term and call them fields everywhere. AFAIK, there's no distinction between the two term except where we currently use them.

We may also want to introduce a term "active field" for whether or not the field can be accessed and padding bytes are padding bytes for the containing type. For example of why this would be useful, how do we describe padding of the field/element in A only exists when the value has the A discriminant?

#[repr(u8)]
enum Enum {
    A((u64, u32)),
    B(u128),
}

Start by auditing the existing uses of “field” and “element” in the Rust Reference and compare them with the enum and tuple example in this issue. Decide whether the terms should be unified and whether “active field” is needed to describe conditional access and padding, then document the agreed terminology and examples.